Feed water heating



Jan. 18, 1955 M. H. KUHNER FEED WATER HEATING Filed NOV. 17, 1951INVENTOR MA); H KUHNER ATTORNEY United States Patent FEED WATER HEATINGMax H. Kuhner, Worcester, Mass., assignor to Riley Stoker Corporation,Worcester, Mass., a corporation of Massachusetts Application November.17, 1951, Serial No. 256,865

2 Claims. (Cl. 122-421) One of the problems which face the manufacturersof steam generating equipment "is that of economizer corr o sion. Thisis particularly true for industrial and central station installationslocated in districts where the fuel to be burned is high in sulphur andwhere at the same time, because of station heat balance, the feed watertemperature is very low. It has been suggested that corrosion is due notso much to the temperature of the combustion gas as it is thetemperature of the water passing through the tubes which producescondensation of sulphur dioxide with resultant corrosion. This isbecause the outer skin temperature of the economizer tubes. only a fewdegrees higher than the temperature of the water within the tubesregardless of what the gas temperature may be. This contention isconfirmed by the fact that external economizer tube corrosion is beingexperienced with installations where the economizer is placed ahead ofan airheater and where the economizer exit gas temperature is over 500F., but where at the same time the temperature of the feedwater enteringthe economizer is in the neighborhood of 200 F. The temperature ofcondensation of flue gas moisture (dew point) rises with the increase ofsulphur in the fuel. The higher the sulphur content the higher is thetemperature at which condensation takes place. It is therefore importantto keep the temperature of the water entering economizer tubes so highthat it is above the condensation temperature of gas to preventcondensation and corrosion. The invention of the applicant obviates thedifiiculties described above and experienced in prior art installations.

It is therefore an outstanding object of the invention to provide meansfor preventing economizer corrosion.

Furthermore, it is an object of the present invention to provide a novelmeans of heating feedwater before its en trance into the economizer of asteam generating unit.

With these and other objects in view, as will be apparent to thoseskilled in the art, the invention resides in the combination of partsset forth in the specification and covered by the claims appendedhereto.

The character of the invention, however, may be best understood byreference to certain of its structural forms, as illustrated by theaccompanying drawings in which:

Figure 1 is a sectional, somewhat schematic view of a steam generatingunit embodying the invention.

Generally speaking, the present invention consists in apparatus forpassing the feedwater through a heat exchanger submerged under theboiler water and installed in one of the boiler drums. Referring toFigure 1 wherein is shown the preferred form of the invention, the steamgenerating unit, generally designated by the reference character 10,comprises a boiler 11 situated in a setting 12. The boiler 11 has anupper steam drum 13 and a lower submerged or water drum 14 joined in awell-known manner by downcomers 15 and risers 16. A spreader stoker 17and a traveling grate 18 provide for fuel burning. A header 19 situatedimmediately above the stoker 17 is joined to the steam drum 13 by waterwall tubes 20. A vertical wall 21 rises above the rear end of the grate18 and serves to define a combustion chamber 22. At least one row ofrisers 16 is situated forwardly of the wall 21 to constitute furtherwater wall tubes. A downwardlyextending vertical wall 23 and anupwardly-extending vertical wall 24 serve to define the convectionpasses of the boiler and baflles 25 serve to cause the hot products ofcombustion to travel in a sinuous path from the combustion chamber 22 toa flue passage 26.

I feedwater temperature rise is approximately 60 "ice In the fluepassage 26 resides an economizer 27 of the return-bend type having tubeloops 28 lying transversely of the path of the gases in the flue passageand having headers 29 lying externally of the flue passage to which thetube loops are connected in a well-known manner.

A conduit 30 is connected with the source of feedwater and to theentrance of a dividing valve 31, which valve may be automaticallycontrolled, as will be described hereinafter. One exit of the valve 31is connected by a conduit 32 to a heat-exchanger 33 situated in thesubmerged drum 14. The heat-exchanger 33 may consist of a number ofcoils of extended-surface tubing as is well-known in the art. The exitend of the heat exchanger 33 is joined to a conduit 34. The other exitof the valve 31 is connected to a two way, hand-operated shut-off valve35 by a conduit 36. The other side of the valve 35 is connected to aconduit 37 which joins with the conduit 34 to merge with a conduit 38.

The valve 31 is connected mechanically to apower relay 39 which in turnis connected electrically to a temperature measuring device such as athermocouple 40 which is situated adjacent one of the last loops of theeconomizer.

The conduit 38 is connected to the first header of the economizer. Aheader next subjacent the first header is connected by a conduit 41 tothe last header. The header just below the header from which conduit 41emerges is connected to the steam drum 13 by means of a conduit 42.

The operation of the apparatus is as follows: the feed water enters theconduit 30 and passes into the dividing valve 31. Some of the feedwaterleaves the valve 31 through the conduit 32, passes through the heatexchanger 33 into the conduit 34. While passing through the heatexchanger 33, the feedwater receives heat from the warm water in thesubmerged drum 14. Some of the feedwater is bypassed by flowing from thevalve 31 into the conduit 36, through the shut-off valve 35 into theconduit 37. The warmed feedwater in the conduit 34 and the by-passedfeedwater in the conduit 37 mingle in the conduit 38 to an intermediatetemperature which should be higher than the dew point of the flue gases.The temperature of the feedwater in the conduit 38 is dependent on thesetting of the dividing valve 31, which in turn is determined by thepower relay 39 responsive to the thermocouple 40. The thermocouple isassociated with the last loop of the economizer since this is theportion where corrosion takes place most readily due to the fact thatthe gas is coolest and the feedwater is coolest in a counter-floweconomizer.

In the preferred embodiment the feedwater passes through the conduit 38into the first loops of the economizer to temper the feedwater evenfurther before it passes through the conduit 41 into the last loop ofthe economizer. The feedwater then proceeds in counterfiow to the gasesin the usual manner before passing through the conduit 42 into the drum13 of the boiler.

The feedwater temperature rise is about 30 F. in a small installationwhere the feedwater temperature is 210 F. and the operating pressure is200 p. s. i. g. at full load of the boiler. In other words, thetemperature of the water entering the economizer will be approximately240 F. with the system of the invention instead of 210 F. Invariably,this is sufficient to prevent condensation. Naturally, a higher watertemperature rise can be obtained with higher operating pressures whenthe temperature of the water contained in the drum is higher.

One of the objections to the system of the invention may be based on theassumption that the heat recovery in the economizer will be impaired bythe higher feedwater temperature entering the economizer. While this maybe true to a certain extent, for the average industrial installation aloss of perhaps one quarter of one percent in overall etficiency will becompensated for by the absence of, or at least greatly reduced, expensefor economizer maintenance and extended boiler outages.

The performance of the heat exchanger is such that a lower feedwatertemperature rise is obtained with the highest rate of feedwater flowcorresponding to the highest rate of boiler steam output. For example,the F. at half load and 40 F. at full load. This characteristic isdesirable because it is usually necessary to maintain a higher feedwatertemperature at part load to prevent condensation of flue gas moisturethan at maximum load when the temperature of the flue gas is naturallyhigher. Itcan also be seen that the heat recovery in the economizer isleast impaired by this scheme during high loads when high efliciency isimportant.

It is obvious that minor changes may be made in the form andconstruction of the invention without departing from the material spiritthereof. It is not, however, desided to confine the invention to theexact form herein shown and described, but it is desired to include allsuch as properly come within the scope claimed.

The invention having been thus described, what is claimed as new anddesired to secure by Letters Patent is:

l. A steam generating unit comprising: a boiler having a steam drum anda water drum, an economizer for heating feedwater before itsintroduction into said boiler, a heat exchanger situated in said Waterdrum, means responsive to the temperature of the section of theeconomizer over which the coolest gases flow for causing a portion ofthe feedwater to flow throughthe heat exchanger before entering theeconomizer to maintain the temperatureof the said section of theeconomizer above the dew point of the gases passing thereover.

2. A steam generating unit having an external source of feedwater,comprising: a boiler having a steam drum and a water drum, an economizerfor heating feedwater before its introduction into said boiler, a heatexchanger situated in said water drum, a three-way dividing valve, atemperature measuring device associated with the section of theeconomizer over which the coolest gases flow, a power relay connected tosaid device and responsive thereto, said power relay being alsoconnected to said dividing valve to change its setting in response tochanges in economizer temperature, said dividing valve being con nectedto the source of feedwater, the heat exchanger, and the economizer forcausing a portion of the feedwater to flow through the heat exchangerbefore entering the economizer to maintain the temperature of the saidsection of the economizer above the dew point of the gases passingthereover.

References Cited in the file of this patent UNITED STATES PATENTS565,986 Hogan Aug. 18, 1896 856,900 Nichols June 11, 1907 877,355 MilneJan..21, 1908 1,453,703 Cederblom May 1, 1923 1,472,474 Jacobus Oct. 30,1923 1,612,854 Broido Jan. 4, 1927 1,753,363 Coghlan et a1 Apr. 8,19301,883,293 Jacobus Oct. 18, 1932 1,938,072 Karlsteen Dec. 5, 19332,070,075 Bessler Feb. 9, 1937 2,623,506 Dalin et al. Dec. 30, 19522,635,587 Dalin et al. Apr. 21, 1953 FOREIGN PATENTS 690,582 GermanyApr. 29, 1940

